Various graphic printers have been described wherein a sheet of paper is positioned on a rotating drum and maintained thereon by means of a vacuum created within the drum. A controlled ink stream is directed at the paper printing the desired pattern thereon. U.S. Patent Application Ser. No. 606,954, filed Aug. 27, 1975, by Hauser et al. and entitled "Paper Handling in Graphic Printer" and having a common assignee with this application and now abandoned describes such an arrangement. As shown in this application, the paper is carried on an apertured drum which is rotated past a print head. The print head moves slowly along the axis of the drum so as to raster scan from top to bottom and from left to right over the surface of the paper. To facilitate insertion and removal of the paper, a shroud is placed very close to the cylindrical surface of the drum and during paper insertion, guides the paper into close proximity with the drum.
A solenoid operated discharge door is provided in the shroud, at a convenient location, and serves to allow the paper to discharge from the drum after release of the vacuum. In response to energization of a solenoid, the door opens and the vacuum within the drum is released. As the paper springs away from the platen due to its natural curl and the centrifugal force exerted thereon, the leading edge of the paper exits from the space between the drum and the shroud, and through the discharge door. It will be appreciated that the speed of release and path of the paper from the drum, after the vacuum has been removed, is related to the weight and stiffness of the paper. Papers of a relatively heavy weight will be released much more quickly than those of a light weight. Such a condition makes it difficult to time the release of the vacuum so that the paper exits directly through the door rather than strikes the edge of the door opening or is forced to travel around the shroud before release. Should the paper be required to travel around the shroud, the centrifugal and frictional force may be insufficient to properly discharge various weight papers. Certain prior arrangements have experienced difficulty in adapting to varying weights of paper. Further, relative humidity effects the stiffness of the paper and may vary the frictional force required to assure that the paper will be discharged from the drum after release of the vacuum. To overcome such conditions for a given weight paper, the vacuum is released when the paper reaches a selected rotational position. The size of the drum is selected so that its circumference is slightly less than the length of the paper so as to provide overlap of the top and bottom edges of the paper.
A light beam is directed tangential to the platen and is interrupted by the paper flap. This light path interruption is sensed by a photocell and the output signal from the photocell is used to control the release of the vacuum in the platen at a selected position of the paper with respect to the exit door. Such a flap sensor is described in U.S. Patent Application Ser. No. 606,959 filed Aug. 22, 1975 by Sokolowski, now abandoned, and entitled "Paper Overlap Edge Sensor" and having a common assignee with this application. Application Ser. No. 606,959 has been replaced by a continuation application Ser. No. 773,246, filed Mar. 1, 1977, now U.S. Pat. No. 4,101,018. Although such an arrangement provides a workable solution for a relatively narrow range of paper weights and relative humidities, the aforementioned problems still exist. Should the user vary the weight of the paper from the weight for which the apparatus has been designed, malfunctioning frequently occurs.
The illustrated embodiment, hereinafter described, readily adapts to a wide range of paper weights without requiring individual paper weight adjustment and thus allows the operator to intermix paper weights as desired. As hereinafter described, paper discharge is accomplished by first releasing the vacuum in the drum to allow the paper to come to rest in the shroud and then reapplying the vacuum to boost the paper out of the discharge opening. Further, since the centrifugal force on the paper, as it is released from the drum, is not relied upon to force the paper out of the shroud, the weight and stiffness of the paper are minor considerations in determining the discharge effectiveness of the device.